The communication between the high speed moving element, typically transportation media for people and/or goods, is supported by a wireless radio system provided with roaming Hand Over (HO) between the network (on-board system) installed on the mobile unit and the network of the ground system.
In the present context, as transportation media it is intended for instance car, trucks, trains for metropolitan or long distance railway lines, tramway cars, trolley buses, ships/boats/rafts, trolley for waste transportation devices etc. The goods can be raw materials, finished or semi-finished products, by-products, wastes etc.
The radio systems actually known to support communications, in all the environments where the speed of the moving media is a characterizing element, like Universal Mobile Telecommunication System (UMTS), Global System for Mobile communications (GSM) or Terrestrial Tracked Radio (TETRA) are limited in terms of bandwidth and supported services and can't be efficiently used when the broadness of the bandwidth characterizing the services to be delivered is an essential requirement.
The above mentioned limitations, in terms of efficiency and grant of the “live” and “real time” attributes to the provided services, are limiting the use of the WLAN (Wireless Local Area Network) technologies defined in the standards IEEE 802.11 FHSS, IEEE 802.11 a,b,g and 802.16, in all cases where the speed of the moving media is the characterizing element. In fact, it is clear that, unlike a home or office environment, where roaming is rare and deferred communication is tolerated, radio communication-based control demands for continuous and efficient communication, complying with the requirements set by the type of services (live, data transmission etc.) to be provided, in an environment where roaming is a certainty and often occurs at very high speeds.
In these circumstances, it would be desirable not only to ensure the connection between the ground control center and the moving vehicle, as in the known systems, but also ensure and maintain the broadband communication channels (tents of Mbit/sec per channel) that allow for simultaneous services such as simultaneous high quality video (25 fps and network latency less than 30 msec) streaming from several cameras with no significant image slowing or freezing phenomena, real time high speed data transmission, real time video broadcasting (news, advertising), real time voice communication.
In greater detail, according to the known art, a network established to provide communications between the subsystems installed on the mobile units and one or more control centers is indicated as Communication System (CS). The CS is an integrated seamless Ethernet-IP network that includes both wire-line and wireless components. The CS is thus a mix of network wire equipment and radiofrequency wireless components, all protected by a safe security system, and is based on commercial off-the-shelf components and open standard software and protocols interconnected and functionally integrated according to architectures and software properly developed as a function of the required applications.
The CS is in practice a combination of hub/switch apparatus, provided with suitable access interfaces (Ethernet or else) and radio, interconnected by optical fiber wire network, copper wires and radio connections. The Ethernet hubs/switches are installed inside equipment rooms and have a dual purpose; to aggregate the interconnection of the Access Point (AP) radio units, and also to form a high-speed Ethernet backbone. The interconnection of the AP to the network switches is achieved via multi/mono mode fiber-optic cabling and electro-optic converters, copper cables and radio connections to establish ground connectivity and/or radio connection. The high-speed Ethernet backbone is achieved by interconnecting the Ethernet/IP switches together via single-mode fiber-optic cabling and/or radio connection.
The AP are typically placed at fixed locations and act as the access interface between the wireless coverage area and the network hubs/switches. The access points for these applications are normally installed in harsh weather conditions and are housed in enclosures which meet the standard established for each specific environment (thermal, vibrations, wind, strength etc).
The on-board network is installed on the mobile media (car, plain, metro train, railway train etc). Depending on the size of the mobile media, the architecture of the mobile network may vary significantly with the target of obtaining the best efficiency out of the wi-fi radio technology used.
The AP locations must provide uniform signal strength over the area of interest. The distribution of APs along the MU path is based on the MU's roaming and joining thresholds, which is based on the determination of the interference/noise floor.
The APs must provide full area coverage with a consistent minimum signal level above the measured noise floor, measured in accordance with the coverage targets and the minimum signal/noise ratios established in order to attain the prefixed design objects (min. and max. established bandwidth, min. and max. throughput etc.)
Once the noise floor has been established it is possible to determine the minimum signal coverage required to assure the system throughput. This is an input to determine the AP positioning (see FIG. 1).
One of the main optimization target, in order to reduce the number of AP's assuring the coverage in the given area is to reduce the radio cells overlapping. among adjacent APs (see FIG. 2).
To this end, it is desired to reduce the pre-handover time (scan and search of a new AP with a better signal) and handover time (disconnection from the AP to which the MU was connected and reconnection to the adjacent AP having a better signal, previously detected by the MU).
The concept of wireless roaming involves therefore a series of MU-to-AP association and connection, disconnection, and re-connection. During the roaming process only the MU is responsible for initiating an association with the AP.
A disconnection between MU and AP occurs when an existing connection disconnects due to the signal level received decreasing below an established threshold, as per the above described criteria. A disconnection may be initiated by either the MU and/or the AP. Re-association occurs when the MU re-associates either with a new AP or a previously associated AP.
At any given instant, a MU may be associated with no more than one AP, this ensuring that the MU maintains only one connection to the network. On the other hand, an AP may have many MUs associated with it at any given time.
The 802.11 specification provides functionality for roaming from one AP coverage area to another AP coverage area. The conventional roaming logic implemented in 802.11 devices is based on an election process, where the precondition for association to the next best AP is based on the MU moving towards a stronger signal while the existing signal is reducing in strength.
The handover procedure can be divided, as mentioned, into two logical steps: discovery and re-authentication, where the device performing the handover is the wi-fi card installed on the CU (Control Unit) on-board the MU.
The discovery (or scan) can be expressed in the following terms. As a result of (e.g.) the train moving along the tracks, the signal strength and signal-to-noise ratio of the link degrades. A handover algorithm, implemented in the wi-fi radio card (RC) installed on the mobile unit starts looking for the new AP performing at the MAC (Media Access Control) layer the active scanning of the selected frequency range.
Then, as far as the re-authentication is concerned, when the wi-fi card finds a new AP the signal of which exceeds a predefined value, a connection to the new AP is allowed.
According to the known art, while in the roaming mode, the mobile MU will select the next best AP from a list of neighboring APs where at least one of these APs will have a signal level above the MU's joining threshold. This roaming logic ensures handovers via networks based on omni-directional cells where the MU may move in any direction and where there is more than one AP to roam to (FIG. 3).
The conventional roaming logic implemented in the 802.11 standard does not provide any guarantee on the time the MU takes to roam to the neighbor cell. The probe delay (scan step) accounts for the biggest part of the overall handover time.
This is one of the main limitation to support the above mentioned real time services especially when the MU is running at high speed. Depending on the speed of the MU the connection can be lost for several seconds (see the self-explanatory diagrams of FIGS. 4 and 5), causing a degradation that in many circumstances can become completely unacceptable with respect to the services that need be guaranteed. In the diagram in FIG. 5 it can be noted in particular the time delay HO which represents the handover duration, and the time range Δ in which, as a result of said delay, there is disconnection and data loss, all the more so when the speed of the vehicle is high.
The lack of control in the handover time is particularly detrimental and critical when, in order to maintain the original characteristics, and this the quality of the transmitted signal, it is necessary to ensure constant and pre-established time intervals between the reception of two successive packets of said signal. Such time interval depends on the type of service, e.g. for the VOIP services it must be less than 100 msec, while for video streaming at 25 fps (frames per second) it must be less than 40 msec. Since said intervals are inclusive of the time the data take to go through the network between the MU and the CC, as a consequence the handover time need be much shorter than 40 msec.